P Block Elements Class 11 Chapter Notes (CBSE Science)

Key Terms for P-Block Elements

P-Block Elements

Elements in which the last differentiating electron enters the outermost p-orbital. Their valence shell electronic configuration is ns²np¹⁻⁶.

Inert Pair Effect

The reluctance of the two s-electrons in the valence shell to participate in chemical bonding, especially in heavier elements of the p-block. This leads to a stable oxidation state that is two units less than the group oxidation state (e.g., +1 for Group 13, +2 for Group 14).

Allotropy

The existence of an element in two or more different physical forms (allotropes) in the same physical state. For example, carbon exists as diamond, graphite, and fullerenes.

Catenation

The property of self-linking of atoms of an element through covalent bonds to form long chains or rings. Carbon exhibits this property to the maximum extent.

Electron Deficient Compounds

Compounds in which the central atom has fewer than eight electrons in its valence shell, making them effective Lewis acids. E.g., Boron trifluoride (BF₃).

Diborane (B₂H₆)

The simplest boron hydride, which has a unique structure containing two 3-center-2-electron (3c-2e) bonds, also known as 'banana bonds'.

Silicones

A group of organosilicon polymers with the repeating structural unit (-R₂SiO-). They are water-repellent, chemically inert, and have high thermal stability.

General Trends in P-Block Elements (Groups 13 & 14)

The p-block elements show significant variation in their properties. The general valence shell electronic configuration is ns²np¹⁻⁶. As we move across a period from left to right, the atomic radius decreases due to an increase in effective nuclear charge. Down a group, the atomic radius increases due to the addition of a new electron shell. However, the atomic radius of Gallium (Ga) is slightly less than that of Aluminium (Al) due to the poor shielding effect of the intervening 10 d-electrons in Ga.

Ionization Enthalpy (IE) generally increases across a period and decreases down a group. There are some irregularities. For example, the IE of Group 13 elements is less than the corresponding Group 2 elements because a p-electron is easier to remove than an s-electron. The trend down Group 13 is also irregular (B > Tl > Ga > Al > In), influenced by size and shielding effects.

Electronegativity first decreases from B to Al and then increases slightly down the rest of Group 13. In Group 14, it decreases from C to Si and is then almost constant. The most significant chemical property is the Inert Pair Effect, which becomes more pronounced for heavier elements like Tl (Thallium) and Pb (Lead), making their lower oxidation states (+1 for Tl, +2 for Pb) more stable than the group oxidation state (+3 and +4, respectively).

Must-Remember Concepts

• General Configuration: The valence shell electronic configuration of p-block elements is ns²np¹⁻⁶.
• Group 13 (Boron Family): Configuration is ns²np¹. Common oxidation states are +3 and +1. Stability of +1 state increases down the group (Al < Ga < In < Tl) due to the inert pair effect.
• Group 14 (Carbon Family): Configuration is ns²np². Common oxidation states are +4 and +2. Stability of +2 state increases down the group (C < Si < Ge < Sn < Pb).
• Anomalous Behavior of First Element: Boron (Group 13) and Carbon (Group 14) show properties different from their congeners due to small size, high ionization enthalpy, high electronegativity, and absence of d-orbitals.
• Lewis Acidity of Boron Halides: The Lewis acid strength is BI₃ > BBr₃ > BCl₃ > BF₃. This is contrary to electronegativity trends and is explained by pπ-pπ back-bonding, which is strongest in BF₃.
• Diborane Structure: B₂H₆ contains two types of hydrogen atoms: four terminal H-atoms (in one plane) and two bridging H-atoms (above and below the plane). The bridging atoms are held by 3-center-2-electron (3c-2e) bonds.
• Allotropes of Carbon: Diamond (sp³ hybridized, tetrahedral, insulator, hardest substance) and Graphite (sp² hybridized, planar hexagonal layers, conductor, soft and slippery).
• Silicones: These are polymers with a repeating (-R₂SiO-) unit, forming a stable Si-O-Si backbone. They are used as sealants, lubricants, and in water-proofing.

Comparison of Diamond and Graphite

Worked Example: Lewis Acidity of Boron Trihalides

• Arrange BF₃, BCl₃, BBr₃ in increasing order of their Lewis acid strength and explain why. Order: BF₃ < BCl₃ < BBr₃ < BI₃ Explanation: A Lewis acid is an electron-pair acceptor. In Boron trihalides (BX₃), the Boron atom is electron deficient (6 valence electrons). To complete its octet, it accepts an electron pair. Based on electronegativity, one would expect F to withdraw electrons most strongly, making BF₃ the strongest acid. However, the opposite is true. This is due to pπ-pπ back-bonding. The halogen atom has lone pairs in its p-orbital, which it can donate back to the vacant p-orbital of Boron. The extent of this back-bonding depends on the size and energy match of the orbitals: In BF₃: The 2p orbital of B and 2p orbital of F are of similar size and energy, leading to effective back-bonding. This reduces the electron deficiency on Boron significantly. In BCl₃: The overlap is between the 2p orbital of B and the 3p orbital of Cl. The size mismatch makes the back-bonding less effective than in BF₃. As we go from F to Cl to Br to I, the size of the halogen's p-orbital increases, making the back-bonding progressively weaker. Therefore, the electron deficiency on Boron increases, and the Lewis acid strength increases accordingly.

Board Exam Traps

A common point of confusion is the trend in atomic radius for Group 13. Remember that Gallium (Ga) has a smaller atomic radius than Aluminium (Al). This is a crucial exception due to the poor shielding by the 10 d-electrons in Ga, which increases the effective nuclear charge. Also, be very precise when explaining the structure of Diborane (B₂H₆). Simply stating it is electron deficient is not enough. You must mention the presence of two 3-center-2-electron (3c-2e) bonds or 'banana bonds' to get full marks.

Quick Revision Check

• What is the inert pair effect? Give an example from Group 14. It is the tendency of the ns² electrons in the valence shell of heavier p-block elements to remain unshared in chemical bonds. For example, in Group 14, Lead (Pb) shows a stable +2 oxidation state in addition to +4.
• Why is diamond an electrical insulator while graphite is a conductor? In diamond, all four valence electrons of each carbon are used in forming strong covalent bonds (sp³ hybridization), leaving no free electrons. In graphite, only three electrons are used in bonding (sp² hybridization), and the fourth electron is delocalized within the layers, allowing for electrical conductivity.
• What is the basic structural unit of silicones? The basic repeating structural unit of silicones is R₂SiO, where R is an alkyl or aryl group.
• Why is CO₂ a gas while SiO₂ is a solid at room temperature? Carbon forms strong pπ-pπ multiple bonds with oxygen, resulting in discrete, linear CO₂ molecules with weak intermolecular forces. Silicon is larger and cannot form effective pπ-pπ bonds; instead, it forms a giant covalent network structure (SiO₄ tetrahedra linked together), which is a high-melting point solid.

Frequently Asked Questions

Frequently Asked Questions